1. Field of the Invention
The invention relates to a method for polishing the edge of a semiconductor wafer.
2. Background Art
Semiconductor wafer conventionally used are usually silicon wafers or a substrate having layer structures derived from silicon (e.g. silicon-germanium). These silicon wafers are used in particular for producing semiconductor components such as memory chips (DRAM), microprocessors, sensors, light emitting diodes and many more.
The quality requirements for the edge of a semiconductor wafer are ever increasing, in particular for large diameters of the semiconductor wafer (diameter≧300 mm). In particular, the edge of the semiconductor wafer should be as far as possible free of contamination and have a low roughness. Moreover, it should be resistant to increased mechanical stresses during handling. The untreated edge of a semiconductor wafer sliced from a single crystal has a comparatively rough and non-uniform surface. It often experiences spalling under mechanical loading and is a source of disturbing particles. It is therefore customary to regrind the edge in order thereby to eliminate spalling and damage in the crystal and to provide it with a specific profile.
Suitable grinding equipment is known from the prior art. It is customary for the semiconductor wafer to be fixed on a rotating table and to be delivered by its edge against the likewise rotating working surface of a machining tool. The machining tools used in this case are generally in the form of discs which are secured to a spindle and have circumferential surfaces serving as working surfaces for machining the edge of the semiconductor wafer. The material-removing grain is usually fixedly anchored in the working surfaces of the machining tools. The grain used normally has a coarse granulation. The grain size is usually specified in mesh in accordance with Japanese Industrial Standard JIS R 6001:1998. An average particle size can be calculated from the mesh figures.
If grinding discs having fine granulation are used, the term fine grinding is often also employed. Such fine grinding discs have a granulation of from 1000 mesh up to 4000 mesh, e.g. those commercially available from Disco Corporation. It emerges during the conversion into particle sizes that, by way of example, 1200 mesh corresponds to an average particle size of 9.5 μm, 5000 mesh corresponds to an average particle size of 2.5 μm and 8000 mesh corresponds to an average particle size of 1.2 μm.
The average particle sizes during fine grinding are from approximately 3 μm up to 10 μm. If grinding discs having coarse granulation are used, the average grain size is greater than 10 μm and usually up to 15 μm. These grinding machining tools are suitable for providing the semiconductor wafer with a rounded edge. It is usual, however, for a certain minimum roughness to remain on the edge surface after the edge rounding.
In a subsequent machining step, the wafer edge that has been ground and treated with an etching medium is therefore usually polished. For polishing, the edge of a centrally rotating semiconductor wafer is pressed against a centrally rotating polishing drum with a specific force (contact pressure). U.S. Pat. No. 5,989,105 discloses an edge polishing method of this type, in which the polishing drum comprises an aluminum alloy and a polishing pad is applied to the polishing drum. The semiconductor wafer is usually fixed on a flat wafer holder, a so-called “chuck.” The edge of the semiconductor wafer projects beyond the chuck, such that it is freely accessible to the polishing drum.
In these customary edge polishing methods, in particular the local geometry in the edge region of the semiconductor wafer is adversely influenced. This is associated with the fact that with the relatively “soft edge polishing pads” used in this case (relatively soft polishing pads to which silica sol is applied are usually used), not only the edge itself but also an outer part on the front and/or rear side of the semiconductor wafer is polished as well, which can be explained by the hard edge “dipping” into the polishing pad to which polishing agent slurry is applied. This has the effect that removal is indeed effected not only in the region of the actual edge but also in the adjoining region on the front and/or rear side.